1. Field of the Invention
The present invention generally relates to low temperature ion implantation, and more particularly to water condensation problems associated with low temperature ion implantation.
2. Description of the Prior Art
Low temperature ion implantation is a new branch of ion implantation. It has been discovered that a relatively low wafer temperature during ion implantation is advantageous for formation of shallow junctions, especially ultra-shallow junctions, which are becoming more and more important for continued miniaturization of semiconductor devices. Besides, the technique also has proven useful for enhancing the yield of ion implantation.
In the beginning of the current low temperature ion implantation process, a wafer is moved from an environment, such as an atmosphere or ambient environment, into an implanter, and is cooled to a temperature lower than a temperature of the environment, such as a temperature lower than the freezing point of water. Herein, the wafer can be cooled at least in a cassette outside of the implanter, in a loadlock of the implanter, in a chamber of the implanter, and so on. After the wafer is cooled, an implanting process is performed in the chamber to implant the cooled wafer. The wafer can be cooled and implanted in the same position or different positions. Moreover, during the implanting process, a temperature adjusting apparatus may be used to keep the temperature of the implanted wafer low enough, such as below the freezing point of water. In general, both the operation of the temperature adjusting apparatus and the practical implantation parameters used by the implanting process are kept constant, such that the implanted result is uniformly distributed over the implanted wafer. After that, the implanted and cooled wafer is moved out of the implanter to the environment for further semiconductor fabrication processing.
However, a serious disadvantage referred to as “water condensation” is associated with the mentioned cooling technique, due to a significant temperature difference between the low temperature ion implantation, usually performed below room temperature, such as at about −15˜−25° C. or even lower, and the temperature of the ambient environment, usually at room temperature, i.e., about 15˜25° C. Therefore, if the implanted and cooled wafer is directly moved from the chamber to the environment after the implanting process is finished, “water condensation” on the surface of the wafer induced by the temperature difference is almost unavoidable. Then, the surface of the wafer must be treated to remove the condensed water before additional semiconductor fabrication processes are performed.
One common solution is to treat the surface of the wafer after the wafer is moved out of the ion implanter and before any following semiconductor fabrication processing, such that the condensed moisture is removed prior to such processing. While this common solution addresses the damage of condensed moisture by removing it after formation, it does not prevent the formation of the condensed moisture from occurring in the first place. Hence, the cost is high, and unpredictable damage to micro-structures of wafers may be unavoidable.
Another common solution is to temporarily locate the implanted wafer inside of the ion implanter, such that the wafer is moved out of the ion implanter only after the wafer temperature has naturally been raised in a vacuum environment. As usual, a long temperature recovery time is required to allow the wafer temperature to be raised from the temperature of the implanting process to the temperature of the external environment. Clearly, this common solution solves the problem of condensed moisture by simply using the vacuum environment to prevent formation of moisture during the period of raising the wafer temperature. Unavoidably, a long temperature recovery time for naturally raising the wafer temperature inside the ion implanter reduces throughput.
Accordingly, there is a need to propose a novel and effective approach to resolve the “water condensation” problem associated with low temperature ion implantation.